Conventionally, there is disclosed an exhaust gas aftertreatment system in which exhaust gas discharged from an internal combustion engine flows through an exhaust pipe; a muffler for reducing exhaust noise that is created when the exhaust gas is released to the atmosphere is connected to the exhaust pipe; a purifier for purifying the exhaust gas is provided in the exhaust pipe; ozone is generated by an ozone generation reactor; the ozone generation reactor and the exhaust pipe are connected to each other on an upstream side of the purifier in a direction in which the exhaust gas flows by an ozone supply pipe; and the ozone generated by the ozone generation reactor is supplied to the inside of the exhaust pipe via the ozone supply pipe (see, for example, Patent Literature 1). In this exhaust gas aftertreatment system, an amount of ozone is detected by an ozone sensor, and this ozone sensor is disposed in at least one of the inside of the ozone supply pipe and immediately before the muffler. Also, the ozone generation reactor is provided with a generation section having a pair of electrodes in the inside thereof, a power source section for applying a high voltage between the electrodes, and an air pump for supplying air into the inside of the generation section. In addition, the ozone supply pipe is provided with an on-off valve which switches communication and interruption between the ozone supply pipe and the exhaust pipe. Furthermore, the power source section, the air pump, and the on-off valve are electrically connected to an ECU (Engine Control Unit), and the exhaust gas aftertreatment system is configured so that their operations are controlled based on an output from ECU.
In the thus configured exhaust gas aftertreatment system, in the case where the ozone sensor is disposed in the inside of the ozone supply pipe, since the ozone supply pipe is not a passage through which the exhaust gas flows, it does not become a high temperature state as in the inside of the exhaust pipe. In consequence, it is possible to use, as the ozone sensor, a semiconductor ozone sensor which is low in heat resistance but is capable of detecting a minute amount of ozone, and the generation amount of ozone by the ozone generation reactor can be accurately detected. Meanwhile, in the case where the ozone sensor is disposed immediately before the muffler, in general, the exhaust gas which has passed through the purifier is cooled during a period in which it flows inside the exhaust pipe, and before the exhaust gas reaches the muffler, it becomes a temperature state such that it is possible to use a semiconductor ozone sensor. In consequence, it is possible to use a semiconductor ozone sensor as the ozone sensor, and the amount of ozone remaining in an unreacted state is accurately detected. Therefore, it has become possible to accurately detect the amount of ozone.
Meanwhile, there is disclosed an ozone generation device in which a high-voltage electrode is provided on one of surfaces of a dielectric; an ground electrode is arranged in parallel via a gap part on the other surface of the dielectric; these electrodes are housed in an ozone generation vessel; and a voltage is applied between the high-voltage electrode and the ground electrode to generate ozone in a raw material gas which has been allowed to flow into the gap part (see, for example, Patent Literature 2). This ozone generation device is configured so that a nitrogen enrichment apparatus concentrates nitrogen in compressed air and removes it, the residual gas after nitrogen has been removed by this nitrogen enrichment apparatus and the above-described compressed air are mixed, and this mixed gas is introduced as the raw material gas into the ozone generation vessel. Also, an oxygen concentration of the raw material gas to be introduced into the ozone generation vessel is detected by an oxygen concentration meter, and a concentration of ozone generated in the ozone generation vessel is detected by an ozone concentration meter. Furthermore, a control means controls the oxygen concentration of the raw material gas to be introduced into the ozone generation vessel, a gas flow rate, and an amount of input power based on respective detection outputs of the oxygen concentration meter and the ozone concentration meter. It is noted that a membrane separation type apparatus is used as the nitrogen enrichment apparatus.
In the thus configured ozone generation device, the nitrogen-concentrated gas (nitrogen-rich gas) is released and removed by the nitrogen enrichment apparatus, and therefore, a large amount of oxygen is contained in the residual gas. Then, the control means is controlled based on the detection output of each concentration meter. Therefore, when the ozone concentration is given as an instruction value, not only the raw material gas is controlled to have optimum oxygen concentration and gas flow rate, but also the amount of input power to the ozone generation device is controlled to be an optimum amount of input power. As a result, the above-described ozone generation device is able to avoid waste of oxygen, is low in costs, and enables the operation efficiency to increase. Also, since a part of the compressed air is enriched by the nitrogen enrichment apparatus, oxygen having a desired high concentration is obtained without increase of the amount of compressed air. As a result, a loss in a compressor that compresses air can be decreased.